Ametek 7230 - Chapter 3 Technical Description; Operating Modes

Technical Description
Chapter 3
3-1
3.1 Introduction
The model 7230 lock-in amplifier is a sophisticated instrument with many
capabilities beyond those found in other lock-in amplifiers. This chapter discusses
the various operating modes provided and then describes the design of the instrument
by considering it as a series of functional blocks. In addition to describing how each
block operates, the sections also include information on the effect of the various
controls.
3.2 Operating Modes
3.2.01 Introduction
The model 7230 incorporates a number of different operating modes which are
referred to in the following technical description, so in order to help the reader's
understanding they are defined here.
3.2.02 Single Reference / Dual Reference
Conventionally, a lock-in amplifier makes measurements such as signal magnitude,
phase, etc. on the applied signal at a single reference frequency. In the model 7230
this is referred to as the single reference mode.
The dual reference mode incorporated in the model 7230 allows the instrument to
make simultaneous measurements at two different reference frequencies, an ability
that previously required two lock-in amplifiers. This flexibility incurs a few
restrictions, most notably that both signals be passed through the same input signal
channel, which implies either that both signals are derived from the same detector
(for example two chopped light beams falling onto a single photodiode) or that they
can be summed prior to measurement, either externally or by using the differential
input mode of the instrument. Nevertheless, the mode will prove invaluable in many
experiments. Note that the restriction that that one reference frequency be from the
internal oscillator and one from an external source which used to apply is removed
for instruments with the latest firmware, allowing dual reference mode operation
with two external reference signals. However, in this case one of the references is
limited to a maximum of 3.0 kHz.
3.2.03 Tandem Demodulation
A further development of the dual reference mode is Tandem Demodulation. In this
mode, the input to the second set of demodulators is taken not from the main ADC as
is the case with normal dual reference mode, but from the filtered X-channel output
of the first set of demodulators. Hence, for example, the mode can be used to
measure the modulation amplitude of an amplitude-modulated “carrier” frequency.
The first set of demodulators operates at the carrier frequency. If the output time
constant of this first stage is short enough, then the X output will represent a signal
at the modulation frequency. The second set of demodulators, this time operating at
the modulation frequency, then measure the amplitude and/or phase of this
modulation.
3.2.04 Single Harmonic / Dual Harmonic
Normally, a lock-in amplifier measures the applied signal at the reference frequency.